Golf ball

ABSTRACT

The invention provides, in a golf ball having a core, an intermediate layer and a cover, a golf ball which combines an intermediate layer formed of an ionomer resin material having a high degree of neutralization with a cover formed of a material that includes a first ionomer resin having a high unsaturated carboxylic acid content and a second ionomer resin having a low unsaturated carboxylic acid content. The golf ball is able to achieve a lower spin rate on full shots with a driver (W#1) without reducing the spin performance on approach shots in the short game, provides high levels of both flight performance and controllability, and also has a good feel at impact.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball provided with a core, at least one intermediate layer, and a cover having numerous dimples on an outer surface thereof. More specifically, the invention relates to a golf ball having an excellent flight performance, an excellent feel at impact, a good durability when repeatedly struck and a good controllability.

In recent years, various performance attributes, including not only distance, but also controllability, durability and feel at impact, have been desired in golf balls. Satisfying all of these attributes with only one type of material is generally difficult. Hence, it is common practice to adopt a construction wherein one, two or more layers (intermediate layer, cover, etc.) are formed around a solid core made of rubber, resin or the like, or around a wound core, with each such layer being given a distinct role. That is, by adjusting the number and thickness of the above layers, the hardness relationships between the layers, and the formulations of the materials making up the respective layers, it has become possible to achieve a performance tailored to the particular desires of the user, such as an emphasis on distance or on controllability.

For example, JP-A 2010-253268 discloses a golf ball which, by optimizing the hardness relationships at the core interior, has been provided with both flight performance and durability. However, ordinary low-hardness ionomers have a rather low acid content of about 10%. Because it is therefore impossible to increase the crosslink density of the material, a sufficient rebound and a spin rate-lowering effect sometimes cannot be obtained. U.S. Pat. No. 7,431,669 discloses a golf ball which uses a high acid-content ionomer in the cover. However, when an ordinary high-acid-content ionomer is used, the spin rate on approach shots decreases, in addition to which a golf ball having a sufficient durability and a good feel cannot be obtained.

In this way, there are generally trade-offs between various performance attributes of a golf ball, such as flight performance, controllability, durability and feel; increasing certain attributes often results in a decline in other attributes. For example, when the cover is formed solely of a material having a high resilience (hard material) in order to increase the flight performance of the ball, the distance traveled by the ball on shots with a driver increases, but the ball is often less receptive to spin on approach shots, resulting in decreased controllability. Moreover, because the cover has less flexibility, it may have a tendency to crack or may, depending on the individual golfer, impart too hard a feel at impact. The golfer generally selects, from the wide variety of balls available, a ball that suits himself or herself according to, for example, the golf course being played, the weather at the time of play, the golfer's own style of play, and personal preferences for feel of the ball at impact. This underscores the importance of developing golf balls which provide a combination of the above-indicated performance attributes and can be enjoyably used by a larger number of golfers.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a golf ball which has a suitable spin rate in the short game and can thus manifest a good controllability, yet which holds down the spin rate on shots with a driver, enabling an excellent flight performance to be achieved, and which moreover provides a good feel at impact.

As a result of extensive investigations aimed at achieving the above objects, the inventors have discovered that, in a golf ball which includes a core, an intermediate layer and a cover, by combining an intermediate layer formed of an ionomer resin material having a high degree of neutralization with a cover formed of a material containing two or more different ionomer resins which include a first ionomer resin having a high content of unsaturated carboxylic acid (acid content) and a second ionomer resin having a low acid content, a lower spin rate can be achieved on full shots with a driver (W#1) without lowering the spin performance on approach shots in the short game, thus enabling high levels of both flight performance and controllability to be achieved and also making it possible to obtain a good feel at impact.

Accordingly, the invention provides the following multi-piece solid golf balls.

[1] A golf ball comprising a core, at least one intermediate layer, and a cover having a plurality of dimples on an outside surface thereof, wherein the intermediate layer is formed of a material comprising:

100 parts by weight of a resin component composed of, in admixture,

-   -   (A) a base resin of (a-1) an olefin-unsaturated carboxylic acid         random copolymer and/or a metal ion neutralization product of an         olefin-unsaturated carboxylic acid random copolymer blended with         (a-2) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester random terpolymer and/or a metal ion         neutralization product of an olefin-unsaturated carboxylic         acid-unsaturated carboxylic acid ester random terpolymer in a         weight ratio of from 100:0 to 0:100, and     -   (B) a non-ionomeric thermoplastic elastomer in a weight ratio of         from 100:0 to 50:50,     -   (C) from 5 to 150 parts by weight of a fatty acid and/or fatty         acid derivative having a molecular weight of from 228 to 1500,         and     -   (D) from 0.1 to 17 parts by weight of a basic inorganic metal         compound capable of neutralizing un-neutralized acid groups in         component A and component C; and the cover is formed of a         material comprising a first ionomer resin having an unsaturated         carboxylic acid content of at least 16 wt % and a second ionomer         resin having an unsaturated carboxylic acid content of 12 wt %         or less, which material has a Shore D hardness of not more than         61.         [2] The golf ball of [1], wherein the cover material has a first         ionomer resin content, based on the total amount of the ionomer         resin components, of 50 wt % or less.         [3] The golf ball of [1], wherein the intermediate layer         material has a degree of neutralization of from 70 to 95 wt %.         [4] The golf ball of [1] further comprising, between the core         and the intermediate layer, an envelope layer which is formed of         a material having a Shore D hardness of not more than 50.         [5] The golf ball of [4], wherein the envelope layer-forming         material is a polyester resin.

BRIEF DESCRIPTION OF THE DIAGRAM

FIG. 1 is a schematic cross-sectional view showing the construction of the golf ball according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described more fully below.

The golf ball of the invention has a solid core, at least one intermediate layer, and a cover. FIG. 1 shows an exemplary construction of the golf ball according to the invention. Referring to FIG. 1, the golf ball G of the invention has a plurality of layers, including at least a core 1, an intermediate layer 2 which encases the core 1, and a cover 3 which encases the intermediate layer 2. The core 1 and the intermediate layer 2 are not limited to single layers, and may each be formed of two or more layers. Although not shown in the diagram, if necessary, the ball may be given a construction in which there is also an envelope layer formed between the core 1 and the intermediate layer 2. In addition, the cover 3 typically has a large number of dimples D formed on the surface thereof to enhance the aerodynamic properties of the ball. Each of the layers is described in detail below.

In this the invention, the solid core may be formed of a known rubber composition. Although not subject to any particular limitation, preferred examples include rubber compositions formulated as shown below.

A rubber material may be used as the primary material in the above core-forming material. For example, the core may be formed of a rubber composition containing, in addition to the base rubber: a co-crosslinking agent, an organic peroxide, an inert filler, sulfur, an antioxidant, an organosulfur compound and the like.

Polybutadiene is preferably used as the base rubber of the rubber composition. It is desirable for this polybutadiene to have a cis-1,4 bond content on the polymer chain of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4 bond content among the bonds on the molecule may result in a lower rebound. Moreover, the polybutadiene has a 1,2-vinyl bond content on the polymer chain of preferably not more than 2 wt %, more preferably not more than 1.7 wt %, and even more preferably not more than 1.5 wt %. Too high a 1,2-vinyl bond content may lower the rebound.

To obtain a molded and vulcanized rubber composition having a good resilience, the polybutadiene used in the invention is preferably one synthesized with a rare-earth catalyst or a Group VIII metal compound catalyst. Polybutadiene synthesized with a rare-earth catalyst is especially preferred.

Such rare-earth catalysts are not subject to any particular limitation. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound with an organoaluminum compound, an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.

In the practice of the invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is particularly advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Suitable examples of such rare-earth catalysts include those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

To increase the rebound, it is preferable for the polybutadiene synthesized using the lanthanide series rare-earth compound catalyst to account for at least 10 wt %, preferably at least 20 wt %, and more preferably at least 40 wt %, of the rubber components.

Rubber components other than the above-described polybutadiene may be included in the rubber composition, insofar as the objects of the invention are attainable.

Illustrative examples of rubber components other than the above-described polybutadiene include other polybutadienes, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject to any particular limitation, are exemplified by the above-mentioned unsaturated carboxylic acids neutralized with desired metal ions. Specific examples include the zinc and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 5 parts by weight, more preferably at least 10 parts by weight, and even more preferably at least 15 parts by weight. The amount included is preferably not more than 60 parts by weight, more preferably not more than 50 parts by weight, even more preferably not more than 40 parts by weight, and most preferably not more than 30 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel at impact, whereas too little may lower the rebound.

The organic peroxide may be a commercially available product, suitable examples of which include Percumyl D (available from NOF Corporation), Perhexa 3M (NOF Corporation), Perhexa C40 (NOF Corporation) and Luperco 231XL (Atochem Co.). The use of one or these alone is preferred.

The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, even more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight. The upper limit in the amount included is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide may make it impossible to achieve a ball having a good feel, durability and rebound.

Examples of suitable inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or as a combination of two or more thereof.

The amount of inert filler included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 5 parts by weight. The upper limit in the amount included is preferably not more than 100 parts by weight, more preferably not more than 80 parts by weight, and even more preferably not more than 60 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a good rebound.

In addition, an antioxidant may be included if necessary. Illustrative examples of suitable commercial antioxidants include Nocrac NS-6, Nocrac NS-30 and Nocrac 200 (all available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly or as a combination of two or more thereof.

The amount of antioxidant included can be set to more than 0, and may be set to an amount per 100 parts by weight of the base rubber which is preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The maximum amount included, although not subject to any particular limitation, may be set to an amount per 100 parts by weight of the base rubber which is preferably not more than 3 parts by weight, more preferably not more than 2 parts by weight, even more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a suitable core hardness gradient, a good rebound and durability, and a spin rate-lowering effect on full shots.

In the practice of the invention, an organosulfur compound may be optionally included in the base rubber in order to enhance the core rebound. In cases where an organosulfur compound is included, the content thereof per 100 parts by weight of the base rubber, may be set to preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight. The upper limit in the organosulfur compound content is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, and even more preferably not more than 2 parts by weight. Including too little organosulfur compound may make it impossible to obtain a sufficient core rebound-increasing effect. On the other hand, if too much is included, the core hardness may become too low, worsening the feel of the ball on impact, and the durability of the ball to cracking when repeatedly struck may worsen.

The rubber composition containing the various above ingredients is prepared by mixing using a typical mixing apparatus, such as a Banbury mixer or a roll mill. When this rubber composition is used to mold the core, molding may be carried out by compression molding or injection molding using a specific mold for molding cores. The resulting molded body is then heated and cured under temperature conditions sufficient for the organic peroxide and co-crosslinking agent included in the rubber composition to act, thereby giving a core having a specific hardness profile. The vulcanization conditions in this case, while not subject to any particular limitation, are generally set to conditions of about 130 to 170° C., and especially 150 to 160° C., for 10 to 40 minutes, and especially 12 to 20 minutes.

The core diameter, although not subject to any particular limitation, may be set to from 30 to 40 mm. In this case, the lower limit is preferably at least 32 mm, more preferably at least 34 mm, and even more preferably at least 35 mm. The upper limit may be set to preferably not more than 39 mm, and more preferably not more than 38 mm.

The deflection of the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), although not subject to any particular limitation, may be set within the range of 2.0 to 6.0 mm. The lower limit is preferably at least 2.5 m, more preferably at least 3.0 mm, and even more preferably at least 3.5 mm. The upper limit is preferably not more than 5.5 mm, and more preferably not more than 5.0 mm. If the core is harder than the above range (smaller deflection), when the ball is struck at a high head speed, a sufficient distance-reducing effect may not be obtained. On the other hand, if the core is softer than the above range (larger deflection), the feel at impact may become too soft and the ball may have a poor durability to cracking when repeatedly struck.

The core has a specific gravity which, although not subject to any particular limitation, may be set in the range of 0.9 to 1.4. The lower limit is preferably at least 1.0, and more preferably at least 1.1. The upper limit may be set to preferably not more than 1.3, and more preferably not more than 1.25.

In this invention, by using the above-described material to form the solid core 1, an increased rebound can be achieved, thus making it possible to provide a golf ball which is capable of obtaining a stable trajectory.

The above core is not limited to a single-layer construction, and may instead have a multilayer construction of two or more layers. By giving the core a multilayer construction, the spin rate on shots with a driver can be reduced, enabling a further increase in distance to be achieved. Moreover, the spin properties and feel of the ball when struck can be further enhanced. In such cases, the core has at least an inner core layer (inner sphere) and an outer core layer.

Next, the materials of the intermediate layer and cover which are formed over the core are described in detail.

First, preferred use may be made of a material containing components A to D below as the intermediate layer-forming material:

100 parts by weight of a resin component composed of, in admixture,

-   -   (A) a base resin of (a-1) an olefin-unsaturated carboxylic acid         random copolymer and/or a metal ion neutralization product of an         olefin-unsaturated carboxylic acid random copolymer blended with         (a-2) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester random terpolymer and/or a metal ion         neutralization product of an olefin-unsaturated carboxylic         acid-unsaturated carboxylic acid ester random terpolymer in a         weight ratio of from 100:0 to 0:100, and     -   (B) a non-ionomeric thermoplastic elastomer in a weight ratio of         from 100:0 to 50:50;     -   (C) from 5 to 120 parts by weight of a fatty acid and/or fatty         acid derivative having a molecular weight of from 228 to 1500;         and     -   (D) from 0.1 to 17 parts by weight of a basic inorganic metal         compound capable of neutralizing un-neutralized acid groups in         component A and component C.

Components A to D are described below.

Component A is the base resin of the intermediate layer-forming material, and contains above components (a-1) and (a-2) in a given ratio.

First, component (a-1) is an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer. In the practice of the invention, the use of a random copolymer that is un-neutralized as component (a-1) is especially preferred from the standpoint of controlling the grafting ratio with the other ingredients.

Here, the olefin in component (a-1) is an olefin in which the number of carbons is generally at least 2, but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid is especially preferred.

Component (a-2) is an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer. In the practice of the invention, the use of a random terpolymer that is un-neutralized as component (a-2) is especially preferred from the standpoint of controlling the grafting ratio with the other ingredients.

Here, the olefin in component (a-2) is an olefin in which the number of carbons is generally at least 2, but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Methacrylic acid is especially preferred.

The unsaturated carboxylic acid ester is exemplified by lower alkyl esters of the above unsaturated carboxylic acids. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, i-butyl acrylate) is especially preferred.

The olefin-unsaturated carboxylic acid random copolymer of above component (a-1) and the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer of above component (a-2) (these are sometimes collectively referred to below as “random copolymers”) can each be obtained by using a known method to copolymerize the above-described olefin, unsaturated carboxylic acid and, where necessary, unsaturated carboxylic acid ester.

It is preferable for the above random copolymers to have controlled unsaturated carboxylic acid contents (acid contents). The content of unsaturated carboxylic acid in component (a-1) may be set to preferably at least 4 wt %, more preferably at least 6 wt %, even more preferably at least 8 wt %, and most preferably at least 10 wt %, and it is recommended that the upper limit be preferably not more than 30 wt %, more preferably not more than 20 wt %, even more preferably not more than 18 wt %, and most preferably not more than 15 wt %. The content of unsaturated carboxylic acid in component (a-2) may be set to preferably at least 4 wt %, more preferably at least 6 wt %, and even more preferably at least 8 wt %, and it is recommended that the upper limit be preferably not more than 15 wt %, more preferably not more than 12 wt %, and even more preferably not more than 10 wt %. If the unsaturated carboxylic acid content in above component (a-1) and/or component (a-2) is too low, the ball rebound may decrease, whereas if it is too high, the processability of the resin material may decrease.

The metal ion neutralization product of the olefin-unsaturated carboxylic acid random copolymer of above component (a-1) and the metal ion neutralization product of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer of above component (a-2) (these are collectively referred to below as “metal ion neutralization products of the random copolymers”) can be obtained by neutralizing some or all of the acid groups on the respective above random copolymers with metal ions.

Illustrative examples of metal ions for neutralizing acid groups in the above random copolymers include Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In the present invention, of these, preferred use may be made of Na⁺, Li⁺, Zn⁺⁺ and Mg⁺⁺; Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially recommended. The degree of neutralization of the random copolymers with the above metal ions is not subject to any particular limitation. These neutralization products may be obtained by a known method. For example, compounds such as formates, acetates, nitrates, carbonates, bicarbonates, oxides, hydroxides and alkoxides of the aforementioned metal ions may be introduced into the above random copolymers.

Commercially available products may be used as above component A. Examples of commercial products which may be used as the random copolymer in above component (a-1) include Nucrel 1560, Nucrel 1214, Nucrel 1035 and Nucrel AN 4221C (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor 5200, Escor 5100 and Escor 5000 (all products of ExxonMobil Chemical). Examples of commercial products which may be used as the metal ion neutralization product of the random copolymer in above component (a-1) include Himilan 1554, Himilan 1557, Himilan 1601, Himilan 1605, Himilan 1706 and Himilan AM7311 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 7930 (E.I. DuPont de Nemours & Co.), and Iotek 3110 and Iotek 4200 (ExxonMobil Chemical). Examples of commercial products which may be used as the random copolymer in above component (a-2) include Nucrel AN 4311, Nucrel AN 4318 and Nucrel AN 4319 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor ATX325, Escor ATX320 and Escor ATX310 (all products of ExxonMobil Chemical). Examples of commercial products which may be used as the metal ion neutralization product of the random copolymer in above component (a-2) include Himilan 1855, Himilan 1856 and Himilan AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 6320, Surlyn 8320, Surlyn 9320 and Surlyn 8120 (all products of E.I. DuPont de Nemours & Co.), and Iotek 7510 and Iotek 7520 (both products of ExxonMobil Chemical). These may be used singly or in combinations of two or more thereof as the respective components.

Either of above component (a-1) and above component (a-2) may be used singly, or both may be used together, as the base resin of the above intermediate layer-forming material. The two components are blended in a weight ratio of component (a-1) to component (a-2) which, although not subject to any particular limitation, may be set to from 100:0 to 0:100, preferably from 80:20 to 30:70, and more preferably from 75:35 to 50:50. At a blending ratio outside of the above range, it may not be possible to obtain a sufficient rebound and durability.

The above-mentioned non-ionomeric thermoplastic elastomer (B) is a component which is preferably included so as to further improve the feel of the golf ball at impact and the rebound. In the present invention, the base resin (component A) and the non-ionomeric thermoplastic elastomer (component B) are sometimes collectively referred to as “the resin component.” Examples of component B include olefin elastomers, styrene elastomers, polyester elastomers, urethane elastomers and polyamide elastomers. In the present invention, to further increase the rebound, it is especially preferable to use an olefin elastomer or a polyester elastomer. A commercially available product may be used as component B. Illustrative examples include the olefin elastomer Dynaron (JSR Corporation) and the polyester elastomer Hytrel (DuPont-Toray Co., Ltd.). These may be used singly or as combinations of two or more thereof.

The amount of component B included, expressed as the weight ratio A:B with above component A, may be set to from 100:0 to 50:50, and preferably from 100:0 to 60:40. If component B accounts for more than 50 wt % of the above resin component, the compatibility of respective components may decrease, which may markedly lower the durability of the golf ball.

The fatty acid and/or fatty acid derivative having a molecular weight of at least 228 which serves as component C is a component which has a very low molecular weight compared with the thermoplastic resin in the above resin component.

It is capable of suitably adjusting the melt viscosity of the mixture, and thus helps in particular to improve the flow properties.

The fatty acid or fatty acid derivative of component C has a molecular weight of at least 228, preferably at least 256, more preferably at least 280, and even more preferably at least 300. The upper limit of the molecular weight is set to not more than 1500, preferably not more than 1000, even more preferably not more than 600, and most preferably not more than 500. If the molecular weight is too low, the heat resistance cannot be improved and the acid group content becomes too high, which may result in a smaller flow-improving effect due to interactions with acid groups present in component A. On the other hand, if the molecular weight is too high, a distinct flow-improving effect may not be achieved.

It is preferable to use as the fatty acid of component C an unsaturated fatty acid containing a double bond or triple bond on the alkyl moiety, or a saturated fatty acid in which the bonds on the alkyl moiety are all single bonds. The number of carbons on one molecule of the fatty acid may be set to at least 18, preferably at least 20, more preferably at least 22, and even more preferably at least 24. The upper limit in the number of carbons may be set to not more than 80, preferably not more than 60, more preferably not more than 40, and even more preferably not more than 30.

Too few carbons, in addition to possibly resulting in a poor heat resistance, may also, by making the acid group content relatively high, lead to excessive interactions with acid groups present in the resin component, thereby diminishing the flow-improving effect. On the other hand, too many carbons increases the molecular weight, as a result of which a distinct flow-improving effect may not be achieved.

Illustrative examples of the fatty acid of component C include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Of these, stearic acid, arachidic acid, behenic acid and lignoceric acid are preferred.

The fatty acid derivative is exemplified by metallic soaps in which the proton on the acid group of the fatty acid has been replaced with a metal ion. Examples of metal ions that may be used in the metal soap include Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of the fatty acid derivative of component C include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred. These may be used singly or as combinations of two or more thereof.

The amount of component C included per 100 parts by weight of the above resin component which includes components A and B may be set to at least 5 parts by weight, preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 18 parts by weight. The upper limit is set to not more than 120 parts by weight, preferably not more than 80 parts by weight, and more preferably not more than 60 parts by weight. If the amount of component C included is too small, the melt viscosity may decrease, lowering the processability. On the other hand, if the amount of component C is too high, the durability may decrease.

In the present invention, use may also be made of, as a mixture of the above-described components A and C, a known metallic soap-modified ionomer (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760, and International Disclosure WO 98/46671).

The basic inorganic metal compound of component D is included for the purpose of neutralizing acid groups in above components A and C. If component D is not included, particularly in cases where a metal-modified ionomeric resin alone (e.g., a metallic soap-modified ionomeric resin mentioned in the above-cited patent publications, alone) is mixed under applied heat, the metallic soap and un-neutralized acid groups present on the ionomer undergo an exchange reaction as shown below, generating a fatty acid. Because this generated fatty acid has a low thermal stability and readily vaporizes during molding, not only does it cause molding defects, when the generated fatty acid deposits on the surface of the molding, it causes a marked decline in paint film adhesion.

In this invention, by including above component D, the acid groups present within components A to C are neutralized, enabling the formation of fatty acids which cause trouble such as molding defects to be suppressed. By thus including component D and suppressing fatty acid formation, the thermal stability of the material increases. At the same time, a good moldability is imparted and the desirable effect of increased resilience as a golf ball material is achieved.

It is recommended that component D be a basic inorganic metal compound, and preferably a monoxide, which is capable of neutralizing acid groups in above components A and C. Because component D has a high reactivity with the ionomeric resin and the reaction by-products contain no organic matter, the degree of neutralization of the material can be increased without a loss of thermal stability.

Illustrative examples of the metal ion used here in the basic inorganic metal compound include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Illustrative examples of basic inorganic metal compounds containing these metal ions include magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. These may be used singly or as combinations of two or more thereof. In the present invention, of the above, a hydroxide or a monoxide is especially recommended. Preferred use may be made of calcium hydroxide and magnesium oxide, which have a high reactivity with component A.

The amount of component D included per 100 parts by weight of the resin component may be set to at least 0.1 part by weight, preferably at least 0.5 part by weight, more preferably at least 1 part by weight, and even more preferably at least 2 parts by weight. The upper limit is not more than 17 parts by weight, preferably not more than 15 parts by weight, more preferably not more than 13 parts by weight, and even more preferably not more than 10 parts by weight. If the amount of component D included is too small, improvements in the thermal stability and resilience may not be obtained. On the other hand, if it is too large, the presence of excess basic inorganic metal compound may have the opposite effect of lowering the heat resistance of the composition.

The mixture obtained by mixing together above components A to D has a degree of neutralization, based on the total amount of acid groups in the mixture, which, although not subject to any particular limitation, may be set to at least 70 mol %, and preferably at least 75 mol %. The maximum degree of neutralization also is not subject to any particular limitation, but may be set to 95 mol % or less. At a degree of neutralization in excess of 95 mol %, the flow properties of the material may decrease, as a result of which molding may become difficult. In the practice of the invention, by highly neutralizing acid groups within the material, even in cases where, for example, a metallic soap-modified ionomeric resin is used, exchange reactions between the metallic soap and un-neutralized acid groups present in the ionomeric resin are less likely to arise during mixture under heating, thereby reducing the risk of declines in the thermal stability, moldability and resilience.

Various additives may be optionally included within the material containing above components A to D. For example, additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be suitably included. These additives are used in amounts which, although not subject to any particular limitation, are generally at least 0.1 part by weight, preferably at least 0.5 part by weight, and more preferably at least 1 part by weight, per 100 parts by weight of the resin component. The upper limit is not more than 10 parts by weight, preferably not more than 6 parts by weight, and more preferably not more than 4 parts by weight.

The above material may be obtained by mixing together the above components A to D under applied heat. For example, the material may be obtained by mixture using a known mixing apparatus, such as a kneading-type twin-screw extruder, a Banbury mixer or a kneader, at a heating temperature of from 150 to 250° C. Alternatively, direct use may be made of a commercial product, examples of which include the products available under the trade names HPF 1000 and HPF 2000 from E.I. DuPont de Nemours & Co.

The method of forming the intermediate layer may be a known method and is not subject to any particular limitation. For example, use may be made of a method which involves placing a prefabricated core in a mold, and injection-molding the material prepared as described above.

The construction of the above-described intermediate layer is not limited to a single layer; if necessary, two or more like or unlike intermediate layers may be formed within the above range. By forming a plurality of intermediate layers, the spin rate when struck with a driver can be further reduced, enabling a larger increase in the distance to be achieved. In addition, the spin properties and feel at the time of impact can be further improved.

The material hardness of the intermediate layer-forming material, although not subject to any particular limitation, may be set to a Shore D hardness of preferably at least 30, and more preferably at least 40. The maximum Shore D hardness, although not subject to any particular limitation, may be set to preferably 65 or less, and more preferably 60 or less. If the intermediate layer is softer than the above range, the rebound of the ball on shots with a driver (W#1) may decrease. On the other hand, if it is too hard the durability of the ball to cracking on repeated impact may worsen. As used herein, “material hardness (Shore D hardness)” refers to the hardness, as measured using a type D durometer in general accordance with ASTM D2240, of a sheet molded under pressure to a thickness of about 2 mm from the material to be measured.

The thickness of the intermediate layer, although not subject to any particular limitation, may be set to preferably from 0.5 to 3.0 mm, and more preferably from 0.8 to 2.0 mm. If the thickness of the intermediate layer is too small, the durability to cracking under repeated impact may worsen, or the ball rebound may decrease, as a result of which a good distance may not be achieved. On the other hand, if the thickness of the intermediate layer is too large, the spin rate on shots with a driver (W#1) may increase, as a result of which a good distance may not be achieved.

The construction of the above-described intermediate layer is not limited to a single layer; if necessary, two or more like or unlike intermediate layers may be formed within the above range. By forming a plurality of intermediate layers, the spin rate when struck with a driver can be further reduced, enabling a larger increase in the distance to be achieved. In addition, the spin properties and feel at the time of impact can be further improved.

Next, the material which forms the cover of the inventive golf ball is described. In the invention, the cover is formed of a material composed primarily of an ionomer resin. The material may be formulated of two or more ionomer resins. In particular, by including the two ionomer resins described below (first and second ionomer resins), a golf ball having a good rebound and an excellent durability can be provided.

The first ionomer resin is a resin having a high unsaturated carboxylic acid content (acid content). The acid content must be at least 16 wt %, and, in particular, is preferably from 18 to 25 wt %.

The first ionomer resin is not subject to any particular limitation, although the use of a metal neutralization product of an olefin-unsaturated carboxylic acid copolymer is preferred.

Here, the olefin in the above copolymer is an olefin in which the number of carbons is generally at least 2, but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are preferred, with the use of methacrylic acid being recommended.

The metal ions which neutralize the above unsaturated carboxylic acid are preferably metal ions selected from among Li⁺, Na⁺, K⁺, Mg⁺⁺, Zn⁺⁺, Cu⁺⁺, Ba⁺⁺, Pb⁺⁺ and Al⁺⁺⁺. From the standpoint of enhancing the scuff resistance, neutralization with Zn⁺⁺ is especially preferred.

The degree of neutralization of the above copolymer based on the total amount of acid groups on the copolymer may be set to from 10 to 100 mol %, and is preferably set to from 20 to 80 mol %.

A commercial product may be used as the first ionomer resin. Illustrative examples include Himilan AM7318, Himilan AM7319 and Himilan AD8547 (all products of DuPont-Mitsui Polychemicals Co., Ltd.)

The second ionomer resin is a resin having a low unsaturated carboxylic acid content (acid content). The acid content must be not more than 12 wt %, and, in particular, is preferably not more than 10 wt %. The minimum acid content, although not subject to any particular limitation, is generally at least 8 wt %.

The second ionomer resin is not subject to any particular limitation, although the use of a metal neutralization product of an olefin-α,β-unsaturated carboxylic acid or an olefin-α,β-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer is preferred, with the use of an olefin-α,β-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer being especially recommended.

Here, the olefin in the above copolymer is an olefin in which the number of carbons is generally at least 2, but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are preferred, with the use of methacrylic acid being especially recommended.

As in the case of the first ionomer resin above, the metal ions which neutralize the unsaturated carboxylic acid are preferably metal ions selected from among Li⁺, Na⁺, K⁺, Mg⁺⁺, Zn⁺⁺, Cu⁺⁺, Ba⁺⁺, Pb⁺⁺ and Al⁺⁺⁺. From the standpoint of enhancing the scuff resistance, neutralization with Zn⁺⁺ is especially preferred.

The degree of neutralization of the above copolymer based on the total amount of acid groups on the copolymer may be set to from 10 to 100 mol %, and is preferably set to from 20 to 80 mol %.

A commercial product may be used as the second ionomer resin. Illustrative examples include Himilan AM7316, Himilan AM7327 and Himilan 1855 (all products of DuPont-Mitsui Polychemicals Co., Ltd.).

In the cover material, the above first and second ionomer resins account for respective ratios of the ionomer resin components which, although not subject to any particular limitation, are preferably set as indicated below.

First, the ratio of the first ionomer resin, although not subject to any particular limitation, may be set to preferably not more than 50 wt %, and more preferably not more than 47 wt %, of the total amount of the ionomer resin components. The minimum ratio, although not subject to any particular limitation, may be set to preferably at least 30 wt %.

The ratio of the second ionomer resin, although not subject to any particular limitation, may be set to preferably at least 20 wt %, and more preferably at least 30 wt %, of the total amount of the ionomer resin components. The maximum ratio, although not subject to any particular limitation, may be set to preferably not more than 70 wt %.

By adjusting within the preferred ranges the ratios in which the above first and second ionomer resin are compounded, a good balance between the spin rate on shots with a driver and the spin rate on approach shots can be obtained.

Hence, in the present invention, by using a mixture of the above first and second ionomer resins as the cover material, a golf ball having a good rebound and an excellent durability can be obtained, and the spin rate on full shots, which is a performance attribute of the ball, can be suppressed. Moreover, the ball is not too hard, and thus is an ideal golf ball having a good feel and a suitable receptivity to spin in the short game.

In addition, various additives may be optionally included in this cover-forming material. For example, additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may be suitably compounded.

The method of forming the above cover may involve, for example, molding the cover by feeding the above material to an injection molding machine and injecting the molten material over the intermediate layer formed as described above. In this case, the molding temperature will differ depending on such factors as the types and blending ratios of the resins, but can generally to set in the range of 150 to 250° C.

It is critical for the cover material to have a Shore D hardness of not more than 61, with a hardness of not more than 59 being especially preferred. The minimum Shore D hardness of the cover material, although not subject to any particular limitation, is preferably set to at least 50, and more preferably at least 53. If this hardness is too low, the spin rate on shots with a driver may increase, as a result of which the shot may describe too high of an arc in flight, resulting in a shorter distance. On the other hand, if the hardness is too high, the ball may be unreceptive to spin in the short game, possibly resulting in a loss of controllability on shots around the green.

In the practice of the invention, by forming the cover so as to be thin, the spin rate-lowering effect on shots with a driver can be increased. The specific thickness of the cover, although not subject to any particular limitation, is preferably from 0.5 to 1.5 mm, more preferably from 0.7 to 1.35 mm, and even more preferably from 0.9 to 1.2 mm. If the cover thickness is too large, the ball may be too receptive to spin on shots with a driver, as a result of which a good distance may not be achieved. On the other hand, if the cover thickness is too small, the ball may be insufficiently receptive to spin on shots in the short game, resulting in a poor controllability, and may have a poor scuff resistance.

The construction of the above-described cover is not limited to a single layer; if necessary, two or more layers may be formed of like or unlike materials. It is preferable in this case to have at least one layer serve as the cover formed of the above-described resin blend. Moreover, it is recommended that the hardnesses and thicknesses of the respective cover layers be adjusted in such a way that these values for the cover as a whole fall within the above-indicated ranges.

In the practice of the invention, by additionally providing a soft envelope layer between the core and the intermediate layer, the feel of the ball at impact can be improved. Examples of materials which may be used in the envelope layer include ionomer resins, urethane resins and polyester resins. In the practice of the invention, by using a polyester resin in particular, a more solid and lively feel can be obtained. Moreover, it is preferable for the envelope layer to be soft. The material hardness, expressed in terms of the Shore D hardness, may be set to preferably not more than 50, more preferably from 30 to 50, and even more preferably from 35 to 45.

In the golf ball of the invention, as in ordinary golf balls, it is preferable for numerous dimples to be formed on the surface of the cover so as to further enhance the aerodynamic properties and thus improve the distance. By optimizing such parameters as the number of dimple types and the total number of dimples, synergistic effects with the above-described ball construction enable a golf ball having a more stable trajectory and an improved distance performance to be obtained. Various treatments such as surface treatment, stamping and painting may be carried on the cover in order to improve the design and durability of the golf ball.

The golf ball of the invention may be made to conform with the Rules of Golf for competitive play, and may be formed to a diameter of not less than 42.67 mm. The weight may be set to generally not less than 45.0 g, and preferably not less than 45.2 g. It is preferable for the upper limit to be set to not more than 45.93 g.

As explained above, the present invention makes it possible to obtain a golf ball which has a lower spin rate on full shots with a driver (W#1) without lowering the spin performance on approach shots in the short game, thus providing high levels of both flight performance and controllability, and which moreover has a good feel at impact.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 5, Comparative Examples 1 to 3 Formation of Core

Solid cores were fabricated by preparing the rubber compositions shown in Table 1 below, then molding and vulcanizing at 155° C. for 15 minutes.

TABLE 1 Material No. 1 No. 2 No. 3 Formulation BR 730 100 100 100 (pbw) Organic peroxide 1.2 1.2 1.2 Zinc oxide 27.19 27.85 27.47 Antioxidant 0.1 0.1 0.1 Zinc acrylate 28.20 26.47 27.62 Organosulfur compound 0.10 0.20 0.00

Details on each of the ingredients mentioned in Table 1 are given below.

-   BR730: A polybutadiene rubber synthesized with a neodymium catalyst,     available from JSR Corporation under the trade name “BR 730” -   Organic peroxide: Available from NOF Corporation under the trade     name “Perhexa 3M” -   Zinc oxide: Zinc oxide, available from Sakai Chemical Co., Ltd. -   Antioxidant: Available from Ouchi Shinko Chemical Industry Co., Ltd.     under the trade name “Nocrac NS-6” -   Zinc acrylate: vailable from Nihon Jyoryu Kogyo Co., Ltd.     Organosulfur compound:

The zinc salt of pentachlorothiophenol

Formation of Intermediate Layer

Next, an intermediate layer was formed by injection-molding the intermediate layer material formulated as shown in Table 2 around the core obtained as described above. In Example 4 alone, prior to forming the intermediate layer, an envelope layer was formed by injection-molding material H.

TABLE 2 Material A B C H Formulation Nucrel AN4319 35 35 35 (pbw) Nucrel AN4221C 65 65 65 Magnesium stearate 60 60 60 Calcium hydroxide 3.5 1 5.2 Hytrel 4047 100 Degree of neutralization (mol %) 87 67 100 —

Details on each of the ingredients mentioned in Table 2 are given below.

-   Nucrel AN4319: An ethylene-methacrylic acid-acrylic acid ester     terpolymer (acid content, 8.0 wt %; ester content, 17.0 wt %)     available from DuPont-Mitsui Polychemicals Co., Ltd. -   Nucrel AN4221C: An ethylene-acrylic acid copolymer (acid content, 12     wt %) available from DuPont-Mitsui Polychemicals Co., Ltd. -   Hytrel 4047: A thermoplastic polyester elastomer available from     DuPont-Toray Co., Ltd.

Formation of Cover

A cover was formed by injection-molding the cover material formulated as shown in Table 3 around the intermediate layer formed as described above, thereby producing a golf ball having an intermediate layer and a cover over a core. At the same time as the cover was formed, a plurality of dimples arranged in identical configurations were formed on the outside surface of the golf balls produced in each of the examples of the invention and the comparative examples.

TABLE 3 Material Acid hardness content Material (Shore D) (wt %) D E F G Formulation Himilan AM7318 64 18 40 70 (pbw) Himilan AM7327 40 9.6 60 30 23 Himilan 1557 59 11 50 Himilan 1855 54 10 50 30 Surlyn AD8547 64 19 47

Details on each of the ingredients mentioned in Table 3 are given below.

-   Himilan AM7318: A sodium-neutralized ethylene-methacrylic acid     copolymeric ionomer, available from DuPont-Mitsui Polychemicals Co.,     Ltd. -   Himilan AM7327: A zinc-neutralized ethylene-methacrylic acid-butyl     acrylate terpolymeric ionomer, available from DuPont-Mitsui     Polychemicals Co., Ltd. -   Himilan 1557: A zinc-neutralized ethylene-methacrylic acid     copolymeric ionomer, available from DuPont-Mitsui Polychemicals Co.,     Ltd. -   Himilan 1855: A zinc-neutralized ethylene-methacrylic acid-butyl     acrylate terpolymeric ionomer, available from DuPont-Mitsui     Polychemicals Co., Ltd. -   Surlyn AD8547: A zinc-neutralized ethylene-methacrylic acid     copolymeric ionomer, available from E.I. DuPont de Nemours & Co.

The golf balls obtained above in Examples 1 to 5 and Comparative Examples 1 to 3 were each evaluated by the methods described below for core deflection, material hardness, flight performance, spin on approach shots, and feel at impact.

Evaluation of Ball Properties (1) Core and Ball Deflection (mm)

The core or ball was placed on a hard plate, and the deflection of each when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured.

(2) Material Hardnesses of Envelope Layer, Intermediate Layer and Cover (Shore D Hardness)

The resin materials for the envelope layer, intermediate layer and cover were formed into sheets having a thickness of about 2 mm, and the hardness was measured with a Type D durometer in accordance with ASTM-2240.

(3) Flight Performance

The total distance of the ball when struck at a head speed of 45 m/s with a W#1 mounted on a golf swing robot was measured. The club used was a TourStage X-Drive 701 driver (loft angle, 10.5°) manufactured by Bridgestone Sports Co., Ltd.

(4) Approach

The spin rate of the ball when hit at a head speed of 20 m/s with a sand wedge (SW) mounted on a golf swing robot was measured. The club used was a TourStage TW-01 manufactured by Bridgestone Sports Co., Ltd.

(5) Feel

Sensory evaluations by ten amateur golfers having head speeds of 35 to 45 m/s when the balls were struck with a driver (W#1) were carried out. The feel was rated according to the following criteria.

Excellent (Exc.): The ball had a good, soft feel Good: The ball had an ordinary feel NG: The ball had a hard feel

(6) Scuff Resistance

A non-plated pitching sand wedge was set in a swing robot, and the ball was hit once at a head speed of 35 m/s, following which the surface state of the ball was visually examined and rated as follows.

Excellent (Exc.): Marks were either not visually confirmed or were inconspicuous Good: Ball could be used again

TABLE 4 Comparative Example Example 1 2 3 4 5 1 2 3 Core Material No. 1 No. 1 No. 1 No. 2 No. 1 No. 1 No. 3 No. 1 Diameter (mm) 37.3 37.3 38.1 35.2 37.3 37.3 37.3 37.3 Deflection (mm) 3.7 3.7 3.7 4 3.7 3.7 3.8 3.7 Envelope Material H layer Thickness (mm) 1.2 Material hardness 40 Intermediate Material A A A A B A A C layer Thickness (mm) 1.45 1.45 1.3 1.3 1.45 1.45 1.45 not moldable Material hardness 55 55 55 55 55 55 55 Cover Material D G D D D E F Thickness (mm) 1.24 1.24 0.99 1.24 1.24 1.24 1.24 Material hardness 56 56 56 56 56 56 62 Ball Diameter (mm) 42.68 42.68 42.68 42.68 42.68 42.68 42.68 Deflection (mm) 3 3 3 3 3 3 3 Performance tests Spin rate, W#1 (rpm) 2,600 2,600 2,530 2,610 2,670 2,650 2,450 Distance, W#1 (m) 230 230 232 230 224 225 234 Spin rate on 5,950 5,950 5,900 5,930 5,950 5,920 5,500 approach shots (rpm) Feel good good good Exc. good good NG Scuff resistance good Exc. good good good Exc. good 

1. A golf ball comprising a core, at least one intermediate layer, and a cover having a plurality of dimples on an outside surface thereof, wherein the intermediate layer is formed of a material comprising: 100 parts by weight of a resin component composed of, in admixture, (A) a base resin of (a-1) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (a-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio of from 100:0 to 0:100, and (B) a non-ionomeric thermoplastic elastomer in a weight ratio of from 100:0 to 50:50, (C) from 5 to 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500, and (D) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in component A and component C; and the cover is formed of a material comprising a first ionomer resin having an unsaturated carboxylic acid content of at least 16 wt % and a second ionomer resin having an unsaturated carboxylic acid content of 12 wt % or less, which material has a Shore D hardness of not more than
 61. 2. The golf ball of claim 1, wherein the cover material has a first ionomer resin content, based on the total amount of the ionomer resin components, of 50 wt % or less.
 3. The golf ball of claim 1, wherein the intermediate layer material has a degree of neutralization of from 70 to 95 wt %.
 4. The golf ball of claim 1 further comprising, between the core and the intermediate layer, an envelope layer which is formed of a material having a Shore D hardness of not more than
 50. 5. The golf ball of claim 4, wherein the envelope layer-forming material is a polyester resin. 